Aliphatic polyesters such as polyglycolic acid or polylactic acid are hydrolyzed in vivo and, in natural environments, are metabolized and degraded to water and carbon dioxide by microorganisms. Therefore, aliphatic polyesters have attracted attention as biodegradable polymeric materials which can be substituted for medical materials or commodity resins. Of these aliphatic polyesters, polyglycolic acid has not only high biodegradability and hydrolyzability when an alkaline solution or the like, for example, is used, but also excellent mechanical characteristics such as heat resistance and tensile strength and, in particular, excellent gas barrier properties when used as a film or a sheet. Therefore, polyglycolic acid is expected to be used as agricultural materials, various packaging (container) materials, or polymeric materials for medical use, and applications have been expanded by using polyglycolic acid alone or combining polyglycolic acid with other resin materials or the like.
A polyglycolic acid can be obtained by polycondensing glycolic acid, but with this method, it is difficult to obtain a polyglycolic acid with a high molecular weight. Therefore, polyglycolic acids with a high molecular weight used as molding materials or the like are often synthesized by performing ring-opening (co)polymerization on a glycolide as a cyclic ester.
That is, a polyglycolic acid can be synthesized by dehydrating and condensing glycolic acid as a monomer. However, with a polycondensation method using glycolic acid as a starting raw material, it is difficult to obtain a polyglycolic acid with a high molecular weight. Therefore, a polyglycolic acid with a high molecular weight is synthesized by performing ring-opening polymerization on a glycolide having the structure of a bimolecular cyclic ester of glycolic acid (also called a “dimeric cyclic ester” hereafter) (that is, 1,4-dioxane-2,5-dione).
In order to mass-produce a polyglycolic acid on an industrial scale using a glycolide as a starting raw material, it is indispensable to efficiently and economically supply high-purity glycolide. However, it was difficult to synthesize glycolide efficiently and economically. Glycolide is a dimeric cyclic ester with a structure in which two molecules of water are eliminated from two molecules of glycolic acid, but when glycolic acids are simply esterified with one another, a low-molecular-weight substance such as an oligomer is ordinarily formed, and it is not possible to obtain a glycolide as a dimeric cyclic ester. Therefore, a method of producing a dimeric glycolide by synthesizing a glycolic acid oligomer and then depolymerizing the oligomer, for example, has been used.
The following is an example of a method conventionally known as a technique for obtaining a dimeric cyclic ester of -hydroxycarboxylic acid such as glycolide.
U.S. Pat. No. 2,668,162 (Patent Document 1) discloses a method of pulverizing a glycolic acid oligomer into a powder form, depolymerizing the ground product by heating to 270 to 285° C. in an ultra-vacuum of from 12 to 15 torr (1.6 to 2.0 kPa) while supplying the powder into a reactor at a ratio of very small increments of approximately 20 g/hour, and then capturing a vapor containing the produced glycolide in a trap. This method can be implemented on a small scale, but it is difficult to increase the scale, and the method is not suitable for mass production. Moreover, with this method, the oligomer becomes heavy at the time of heating and remains inside the reactor as a large residue, so the yield is low and the residue cleaning operation is complex. Further, with this method, a glycolide with a high melting point is deposited on the inside wall of the recovery line together with by-products, which may plug the line, and the recovery of the deposits in the line is also difficult.
U.S. Pat. No. 4,835,293 (Patent Document 2) and U.S. Pat. No. 5,023,349 (Patent Document 3) disclose a method of heating an -hydroxycarboxylic acid oligomer to form a melt, blowing an inert gas such as nitrogen gas onto the surface of the melt, making the cyclic ester that is produced and volatilized from the surface accompany the gas flow, and then recovering the product. With this method, the production rate of the cyclic ester is small, and since a large amount of an inert gas is blown onto the melt, the production cost becomes high due to the need to preliminarily heat the inert gas. Further, with this method, an increase in weight progresses in the oligomer melt during heating, and a large amount of heavy material remains inside the reaction canister as a residue, so the yield is low and it is complicated to clean the residue.
French Unexamined Patent Application Publication No. 2692263A (Patent Document 4) discloses a method of adding an oligomer of an -hydroxycarboxylic acid or an ester or salt thereof to a solvent containing a catalyst and stirring while heating to achieve catalytic decomposition. This method is performed at normal pressure or increased pressure using a solvent suitable for accompanying a cyclic ester in the gas phase, and the gas is condensed to recover the cyclic ester and the solvent. In this document, an example using a lactic acid oligomer and dodecane (boiling point: approximately 214° C.) as a solvent is illustrated. However, when the present inventors conducted additional tests under the same conditions using a glycolic acid oligomer and dodecane, the formation of heavy material progressed simultaneously with the initiation of the depolymerization reaction, and the production of glycolide stopped at a point when only a very small amount of glycolide had been produced. Moreover, the reaction residue was viscous, and cleaning required a substantial amount of labor.
Japanese Unexamined Patent Application Publication No. H9-328481A (Patent Document 5) discloses a method of using a high-boiling-point polar organic solvent in a method for producing a dimeric cyclic ester of -hydroxycarboxylic acid by depolymerizing an -hydroxycarboxylic acid oligomer. This production method is a method of heating a mixture containing from 30 to 5,000 parts by weight of a high-boiling-point polar organic solvent per 100 parts by weight of an -hydroxycarboxylic acid oligomer to a temperature at which depolymerization occurs so as to form an essentially uniform solution phase, further continuing heating at the same temperature to distill out the dimeric cyclic ester that is produced together with the high-boiling-point polar organic solvent, and then recovering the dimeric cyclic ester from the distillate. With this method, it is possible to obtain a dimeric cyclic ester from an -hydroxycarboxylic acid oligomer with high yield while preventing the oligomer from becoming a heavy material.
Patent Document 5 cites multiple polar organic solvents with a boiling point within the range of from 230 to 450° C. as high-boiling-point polar organic solvents, but the solvents which were specifically used and in working examples and for which effects were confirmed are the aromatic ester compounds di(2-methoxyethyl)phthalate, diethylene glycol dibenzoate or benzyl butyl phthalate, dibutyl phthalate, and tricresyl phosphate. When the present inventors further investigated depolymerization reactions using these aromatic ester compounds as high-boiling-point polar organic solvents, it became clear that when heated for a long period of time to a temperature at which the depolymerization of a glycolic acid oligomer occurs, the aromatic ester compounds tend to cause thermal degradation. When an aromatic ester compound thermally undergoes thermal degradation, a purification step becomes necessary to reuse the compound as a solvent. In addition, in a depolymerization reaction, a necessity arises to add an amount corresponding to the amount of the degraded aromatic ester compound. As a result, it is difficult to further reduce the production cost of a dimeric cyclic ester.
Further, there have been practically no proposed conventional methods for producing a glycolide primarily using a glycolic acid oligomer as a starting raw material for depolymerization and using a polyglycolic acid with a high molecular weight. Japanese Unexamined Patent Application Publication No. 2000-119269 (Patent Document 6) proposes a method for producing a glycolide in which a polyglycolic acid is subjected to solid-phase depolymerization within a temperature range of 200° C. or higher and less than 245° C. However, this method is not necessarily suitable as a method for mass-producing a glycolide efficiently on an industrial scale. In addition, with this method, the polyglycolic acid tends to become heavy when the heating temperature is not strictly controlled.
The present inventors have proposed a method for producing a cyclic ester as WO/2002/14303 (Patent Document 7), wherein:
(I) a mixture containing an aliphatic polyester and a polyalkylene glycol ether, which is expressed by the following formula:X1—O—(—R1—O—)p—Y(wherein R1 is a methylene group or a straight-chain or branched alkylene group having from 2 to 8 carbon atoms, X1 is a hydrocarbon group, Y is an alkyl group or an aryl group having from 2 to 20 carbon atoms, p is an integer of 1 or greater, and when p is 2 or greater, a plurality of R1 moieties may be the same or different from one another);and has a boiling point of from 230 to 450° C. and a molecular weight of from 150 to 450, is heated to a temperature at which the depolymerization of the aliphatic polyester occurs at normal pressure or reduced pressure;(II) a substantially uniform solution phase is formed in which a melt phase of the aliphatic polyester and a liquid phase consisting of the polyalkylene glycol ether;(III) heating is continued in the solution state so as to produce the cyclic ester by depolymerization and distill out the cyclic ester together with the polyalkylene glycol ether; and(IV) a cyclic ester is recovered from the distillate. According to this method, the cyclic ester formed by depolymerization is distilled off together with the polyalkylene glycol ether and both compounds are separated into distinct liquid phases to recover the cyclic ester phase, while the polyalkylene glycol ether phase without thermal deterioration may be circulated to the reaction system of depolymerization for its reuse. In the method described in Patent Document 7, a polyalkylene glycol ether is used at a ratio of ordinarily from 30 to 500 parts by weight and preferably from 50 to 200 parts by weight per 100 parts by weight of the aliphatic polyester, and in a specific example, from 450 to 1,000 parts by weight of a polyalkylene glycol ether is used per 100 parts by weight of the aliphatic ester. Therefore, in order to distill out the cyclic ester produced by depolymerization together with the polyalkylene glycol ether, a large amount of thermal energy is required, so there has been a demand for further improvements.
Polyglycolic acid is expected to be mass-produced and used in large quantities in the future, and the recycling of the product waste will be a critical issue. The recycling of mold wastes produced as a by-product at the time of the molding of polyglycolic acid will also become an issue. If a glycolide could be produced by depolymerizing a polyglycolic acid, it would become easy to recycle the polyglycolic acid.